1. Technical Field
The present invention relates to a liquid crystal display (LCD) device, and particularly relates to an LCD device of an active matrix type in an In-Plane Switching (IPS) mode of a high aperture ratio and high contrast.
2. Background Art
A display of a twisted nematic (TN) system with high contrast is widely used. However, since molecular axes of liquid crystal (LC) molecules rise by a vertical electric field, a display device of the TN system includes a significant viewing angle dependency. In recent years, as demand for a large monitor of a TV is increasing, an IPS mode becomes widespread. In a display device of the IPS mode, molecular axes of LC molecules rotate by a horizontal electric field in a plane parallel to a substrate to perform display. Since the IPS mode does not include viewing angle dependency over a rising angle of the molecular axes, viewing angle characteristics thereof is substantially more advantageous than that of the TN system.
In a display device of the IPS mode, pixel electrodes and common electrodes are arranged in a shape like a comb teeth, and a horizontal electric field is applied between the pixel electrodes and the common electrodes. For this reason, a ratio of an electrode area to display areas is large. That is, the display device of the IPS mode includes a low aperture ratio. Since the display device of the IPS mode is driven by a horizontal electric field, LC molecules in display areas tend to be affected by an electric field leaked from video signal wiring lines and a vertical cross talk easily occurs.
For example, a solution for such problem is disclosed in Japanese Patent Application Laid-Open No. 2002-323706 (patent document 1). FIG. 35A shows one pixel plan view and FIG. 35B shows a cross sectional view along lines I-I, II-II and III-III. A plurality of scan signal wiring lines 3501, which are first metal layers, and two common signal wiring lines 3502 in parallel thereto are formed on a substrate. A first insulating film 3503 is formed on the plurality of scan signal wiring lines and the plurality of common signal wiring lines. A plurality of video signal wiring lines 3504, which are second metal layers, a thin film transistor (TFT) 3505 and source electrodes 3506 are formed on the first insulating film. The source electrodes 3506 are disposed at both sides of a plurality of pixels, and are connected to pixel auxiliary wiring lines 3506B that are located in the same layer as the source electrodes. The respective source electrodes 3506 form a storage capacitance in areas overlapped the plurality of common signal wiring lines 3502. The source electrodes 3506 and the plurality of common signal wiring lines 3502 are patterned like a saw shape.
In edges in display areas, the saw-like pattern portions suppress an electric field which causes a reverse-rotation of LC molecules. A second insulating film 3507 is formed on the plurality of video signal wiring lines 3504, the TFT 3505 and the source electrodes 3506. A third insulating film 3508 that is a transparent insulating film is formed on the second insulating film 3507. Pixel electrodes 3509 and common electrodes 3510 which are transparent electrodes are formed on the third insulating film 3508. The plurality of video signal wiring lines 3504 are completely covered by the common electrodes 3510 in a wiring line width direction via the second insulating film 3507 and the third insulating film 3508. The pixel electrodes 3509 and the common electrodes 3510 are electrically connected to the source electrodes 3506 and the plurality of common signal wiring lines 3502 respectively via contact holes 3511 and 3512.
The pixel electrodes 3509 and the common electrodes 3510 which are arranged in a shape of comb teeth are transparent electrodes. Thus, areas on the electrode contribute to transmittance. According to a simulation, contribution to the transmittance of the transparent electrodes increases an effective aperture ratio by about 8%. Since areas on the plurality of video signal wiring lines are completely covered by the common electrodes in the wiring line width direction, it is possible to extend an opening to areas near the plurality of video signal wiring lines. Thus, reverse-rotation of liquid crystal molecules is prevented in edges in the display areas, and efficiency for light utilization rises to a maximum extent.
Leaked electric fields from the plurality of video signal wiring lines are shielded by the common electrodes. Accordingly, vertical cross talk decreases. Further, although load capacity occurs between the plurality of video signal wiring lines and the common electrodes, the load capacity does not influence drive of the display device because of an insulating film having low dielectric constant.
A solution to the above problem is also disclosed in Japanese Patent Application Laid-Open No. 2003-207803 (patent document 2). FIG. 37A shows a plan view of one pixel and FIG. 37B shows a cross sectional view along lines I-I, II-II and III-III. A plurality of scan signal wiring lines 3701 that are first metal layers, and two common signal wiring lines 3702 in parallel thereto are formed. A first insulating film 3703 is formed on the plurality of scan signal wiring lines 3701 and the plurality of common signal wiring lines 3702. A plurality of video signal wiring lines 3704 that are second metal layers, a TFT 3705 and source electrodes 3706 are formed on the first insulating film. Although source electrodes 3706 are provided at both sides of a plurality of pixels in FIG. 37A, the source electrodes are not connected to each other in the same layer. The respective source electrodes 3706 are connected electrically via contact holes 3711, 3713 and pixel electrodes 3709. The respective source electrodes 3706 form a storage capacitance in the areas overlapped the plurality of common signal wiring lines 3702.
The source electrodes 3706 and the plurality of common signal wiring lines 3702 are patterned like a saw shape. In edges in display areas, the saw-like pattern suppresses an electric field which causes reverse-rotation of LC molecules. A second insulating film 3707 is formed on the plurality of video signal wiring lines 3704, the TFT 3705 and the source electrodes 3706. A third clear insulating film 3708 is formed on the second insulating film 3707. The pixel electrodes 3709 and common electrodes 3710 which are transparent electrodes are formed on the third insulating film 3708. The plurality of video signal wiring lines 3704 are completely covered by the common electrodes 3710 in a wiring line width direction via the second insulating film 3707 and the third insulating film 3708. The pixel electrode 3709 and common electrodes 3710 are electrically connected to the source electrodes 3706 and the plurality of common signal wiring lines 3702 respectively via contact holes 3711, 3712 and 3713.
In recent years, a liquid crystal display (LCD) device with high definition is required. In the patent document 1, high-definition LCD is not realized. The second metal layer is illustrated on FIG. 36A. When the plurality of video signal wiring lines 3604 and the pixel auxiliary wiring lines 3606B become close in the same layer, a foreign particle or the like tend to cause short-circuiting therebetween. FIG. 36B shows an example in which the two wiring lines short-circuit. The plurality of video signal wiring lines 3604 and the pixel auxiliary wiring lines 3606B are connected through a leak pass 3606C. Electric potential of the pixel electrode is influenced by change of electric potential of video signal wiring lines 3604 in this state. A leaked pass looks like a bright point on a dark screen and looks like a dark point on a light screen. Hereinafter, such a point defect which performs like above is called “a leak bright point”. A demand to the image quality is increased in recent years, and in particular, a display device without the leak bright point is strongly required.
In the patent document 2, a contact hole 3713 is disposed and two source electrodes 3706 are connected via a transparent pixel electrode 3709. By eliminating pixel auxiliary wiring lines in the same layer, short-circuiting to a plurality of video signal wiring lines in the same layer is decreased. However, even in such configurations, the source electrodes and a plurality of video signal wiring lines highly tend to short-circuit. FIG. 38A shows only a second metal layer. Because a storage capacitance is formed between the source electrodes 3806 and the plurality of common signal wiring lines 3802, the source electrodes 3806 must be arranged so as to be more adjacent to the plurality of video signal wiring lines 3804 than the pixel auxiliary wiring lines. FIG. 38B shows that the plurality of video signal wiring lines 3804 and the source electrodes 3806, which are formed in the same layer, make short-circuiting through a leaked pass 3806C. In such configuration, potential of the pixel electrode is influenced by change in electric potential of the plurality of video signal wiring lines 3804, and the leak bright point occurs as stated in the patent document 1. By removing the pixel auxiliary wiring lines, short-circuiting in the same layer is reduced to some extent compared with the patent document 1. However, substantial reduction is not realized.
The display devices in the related art display form a storage capacitance by overlapping between the plurality of common signal wiring lines and the source electrodes. In the display device, short-circuiting between the plurality of video signal wiring lines and the source electrodes is still likely. Thus, improvement of substantial yield is difficult in the related art.